Decode as LUA - how do I do that?

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I am using a third party LUA dissector (which I know works, as I have previously used the same file successfully with Wireshark a few months ago).

Unfortunately, this time around the decode does not happen, and the TCP payload is just shown as the raw hex data, instead of parsed and decoded values as provided by the dissector file.

I seem to remember that last time round, I set Decode As... to use the LUA dissector for the specific port, but this time round the LUA dissector does not appear in the "Decode As" list of protocols, and I can see no way of adding it, so I can't set it for the specific port I need to decode.

I'd upload a screenshot but apparently I need 60 points to do so. Both the protocol (MAVLINK_PROTO) and the LUA_Dissector appear in the Internals>Supported Protocols, and it's enabled under Enabled Protocols, and I've checked that a LUA file runs using -X lua_script:hello.lua, as suggested in the wireshark docs.

Can anybody suggest what else I need to do?

I'm running wireshark-GTK 2.2.6 on Ubuntu 16.04.

Thanks, Stevod